Known in the art is a multi-stage process for producing hydrolyzates of a starch-containing feedstock (cf. U.S. Pat. No. 4,062,728; Cl. 195-31, 1977).
The process comprises disintegration of the starch-containing feedstock, liquefaction of starch in the presence of .alpha.-amylase at a temperature of from 100.degree. to 110.degree. C. for 3 minutes and then also for 3 minutes at a temperature of 140.degree. C. under a pressure sufficient to provide for the liquid state of the starch-containing feedstock. The resulting liquefied suspension of starch is subjected to instant evaporation in the atmosphere thus reducing the pressure to atmospheric and causing cooling of the suspension.
Thereafter the suspension is subjected to repeated liquefaction under atmospheric pressure at a temperature of from 90.degree. to 100.degree. C. in the presence of an additional amount of .alpha.-amylase.
Also known in the art is a process for producing hydrolysates of a starch-containing feedstock for a wort (cf. Japanese Patent Application No. 1,540,552 published Feb. 14, 1979).
According to this process, barley is gelatinized and, when required, liquefied by the fermentative method by heating under pressure, for example in water or in an extruder, whereafter the gelatinized product is subjected to fermentative hydrolysis in the presence of .alpha.-amylase at a temperature within the range of from 50.degree. to 60.degree. C.
The gelatinization process does not ensure a 100% conversion of starch from the feedstock to the soluble state.
Also known is a process for producing hydrolysates of a starch-containing raw material for alcoholic fermentation and an apparatus therefor (cf. D. N. Klimovsky, V. A. Smirnov, V. N. Stabnickov "Alcohol Technology", Moscow, 1967).
In this process the purified and weighed starch-containing feedstock, e.g. grain, is transported, by means of a conveyor, to disintegration, whereupon it is milled to a 100% passage thereof through a sieve with a cell of 1 mm.sup.2. The disintegrated grain is fed to a mixer, into which water is also supplied to form a kneading mass. Then the grain kneading mass is fed to the heat-treatment - cooking of the feedstock. The cooking is effected in three series-connected apparatus: pre-cooker, cooking column and after-cooker.
The pre-cooker comprises a horizontal cylindrical vessel provided with a stirrer. A steam inlet pipe is mounted in its lower section. The grain kneading mass is hetaed here to a temperature of 80.degree.-85.degree. C. and kept for 4-5 minutes.
From the pre-cooker the kneaded grain mass is fed to a cooking column, wherein it is heated to a temperature of from 120.degree. to 150.degree. C. by steam admitted under a pressure of from 3 to 4 atm.
From the cooking column the mass overflows into an after-cooker, wherein it is additionally cooked at a temperature of from 140.degree. to 150.degree. C. without the supply of live steam.
The duration of residence of the mass in the cooking column is 25 to 30 minutes, in the after-cooker 25-30 minutes so that the total residence time is 50-60 minutes.
After the heat-treatment the feedstock is cooled to a temperature of 58.degree.-60.degree. C. and delivered into an apparatus for the fermentative hydrolysis, into which an aqueous enzyme solution is added in a ratio of 1:3-4.
The hydrolysis is conducted for 50-60 minutes under stirring, effected for example by an impeller. In this case an intensive stirring of the solution is not ensured, dead zones are formed in the apparatus, thus causing a non-uniform distribution of the solution in the apparatus.
During the heat-treatment of the starch-containing feedstock there are formed non-condensed gases and volatile organic substances which cause corrosion of the apparatus and, as a result, impair the quality of the produced food-grade ethanol. The high temperature employed in the heat-treatment of the starch-containing feedstock contributes to an increased rate of corrosion of the process equipment.
The heat-treatment of the feedstock at a temperature corresponding to the used pressure of saturated steam is associated with increased losses of the starch contained in the feedstock due to acceleration of the sugar-amine reaction. An excessive amount of the extra-steam is formed which is difficult to utilize. Furthermore, the output of the alcohol from a unit weight of the feedstock is reduced, while the rate of power consumption for the heat-treatment of the feedstock is increased.
These disadvantages result in a reduced service life of the equipment for the heat-treatment of the starch-containing feedstock and, in certain cases, even in a breakdown.
Further disadvantages are such as a long duration of the process of cooking of the feedstock under a high pressure (up to 60 minutes) necessitating a large volume of the process equipment and a large floor area occupied thereby.